| Name | CR | Type | Environment |
| Obfuchsia | 12 | plant | any vacuum |
| Oblivion Shade | 8 | undead (incorporeal) | any |
| Odheo | 1 | vermin | any underground or urban |
| Olghuth | 10 | aberration (aquatic) | cold and temperate aquatic (Aucturn) |
| Oma | 16 | magical beast | any vacuum or gas giant |
| Omduveng | 10 | magical beast | any (Domdun’s Bilestone) |
| Oni, Ja Noi | 5 | outsider (goblinoid, native, oni, shapechanger) | any |
| Oni, Kanabo Commando | 3 | outsider (goblinoid, native) | any |
| Oni, Oni, Spirit | 2 | outsider (native, oni) | any |
| Oni, Tsabagu | 10 | outsider (native, oni, shapechanger) | any |
| Onkushi | 13 | outsider (extraplanar, fire, good) | any (Plane of Fire) |
| Oracle of Oras | 13 | plant | any |
| Orc, Orc Technician | 1/2 | humanoid (orc) | any (Apostae) |
| Orc, Orc Trooper | 5 | humanoid (orc) | any (Apostae) |
| Orocoran | 6 | aberration | any (Aucturn) |
| Orocoran, Orocoran Ichor Lord | 9 | aberration | any (Aucturn) |
| Osharu | 1/2 | monstrous humanoid | any |
| Osharu, Osharu Headteacher | 8 | monstrous humanoid | any |
| Osifern, Enigma | 8 | animal (aquatic) | aquatic (any water) |
| Osifern, Sprout | 1/2 | animal (aquatic) | aquatic (any water) |
| Ossiworm Agent | 10 | undead (shapechanger) | any |